Laser-assisted chemical etch (LACE) has been used to prepare structures useful in micro-electrical mechanical systems (MEMS), nano-electrical mechanical systems (NEMS), and microfluidics. LACE may involve exposing a structure that includes a substrate and a film to a chemical etchant, such as chlorine gas (Cl2), and to light. The light preferentially enhances the reaction etchant with respect to the substrate, but not with respect to the film. As such, the substrate may be selectively etched in regions exposed to the light and to the chemical etchant, thus creating a cavity that frees the film from the substrate in a selected region.
However, only selected types of structures have been prepared thus far using LACE. For example, FIGS. 1A-1B illustrate cross-sectional views of exemplary structures formed using the techniques described in U.S. Pat. Nos. 7,419,915 and 7,419,917 to Abraham, the entire contents of each of which are incorporated by reference herein. FIG. 1A illustrates a precursor structure including bulk silicon (Si) substrate 110 and patterned silicon dioxide (SiO2) layer 120, which is buried within Si substrate 110, and formed using ion implantation. SiO2 layer 120 may be partially freed from Si substrate 110 by simultaneously exposing the substrate to an etchant such as chlorine (Cl2) gas (not illustrated) and to a laser beam. Abraham discloses that such exposure locally heats volumes of Si substrate 110 about SiO2 layer 120, preferentially etching away portions of substrate 110 to form cavity 130 about the SiO2 layer 120.
Other references disclose forming channels in substrates beneath surface films, also by exposing such structures to an etchant and to light. However, the structures prepared using such methods are limited in the type of channel that may be formed and/or the quality of the surface film remaining after processing. For example, U.S. Pat. No. 5,662,814 to Sugino discloses using Cl2 gas and ultraviolet light to etch an Si substrate having a porous SiO2 film on its surface. The Si substrate is etched anisotropically via the pores in the SiO2 film; that is, certain crystallographic planes of the Si substrate are preferentially etched, resulting in hollows in the substrate having shapes that depend on the crystal structure of the Si, such as pyramidal hollows. The pores in the SiO2 film are subsequently covered using another film.
U.S. Pat. No. 6,878,567 to Winer discloses patterning an SiO2 or silicon nitride layer on top of an Si substrate so as to have access holes. The Si substrate is exposed to Cl2 gas via the access holes, and a laser beam used to mill channels in the substrate between the access holes. Thus, although the channels of Winer are not limited to anisotropically etched hollows, as in Sugino, the techniques of both references rely on the use of pores or access holes in the surface films overlying the substrate. Such pores or access holes may limit not only the quality and strength of the surface layer, but also the types of applications for which the final product can be used. Also, the available film compositions are limited.